A terahertz region with a wavelength of 15 μm to 300 μm (0.1 THz to 20 THz) is a region in which a semiconductor laser such as a quantum cascade laser has difficulty in operating at room temperature. A DFG-QCL beam source that outputs terahertz waves by difference frequency generation (DFG) in a middle infrared quantum cascade laser (QCL) oscillating in two wavelengths is known as a small terahertz (THz) light source which can operate at room temperature. In the DFG-QCL beam source, a stacked body that outputs terahertz waves is stacked on an Fe-doped (semi-insulating) indium phosphide (InP) single-crystal substrate. In general, there is a large absorption coefficient in a terahertz frequency band in Group III-V compound semiconductors such as InP and GaAs. Accordingly, in such a DFG-QCL beam source, there is a problem in that the Fe-doped (semi-insulating) InP single-crystal substrate can easily absorb terahertz waves and thus the output of terahertz waves decreases.
In this regard, a non-patent document (Optica p. 38-43, vol. 4, No. 1, January 2017 (Jung et al.,)) discloses a technique of removing an Fe-doped (semi-insulating) InP single-crystal substrate and employing a silicon (Si) substrate as a new substrate in the process of manufacturing a DFG-QCL beam source. A Si substrate less likely to absorb terahertz waves than an Fe-doped (semi-insulating) InP single-crystal substrate. Accordingly, it is possible to improve extraction efficiency of terahertz waves in a DFG-QCL beam source.